Spin and charge susceptibilities of the two-orbital model within the cluster perturbation theory for Fe-based materials

Cluster perturbation theory is used to calculate band structure, spectral functions, Fermi surface, and spin and charge susceptibilities for the two-orbital model of iron pnictides with the on-site multiorbital Hubbard interactions. Susceptibilities are calculated within the approximation combining the cluster perturbation theory for the self-energy corrections and the random-phase approximation (RPA) for the vertex renormalizations. Calculations for the small values of Hubbard repulsion $U \leq 2$ confirm that the rigid band approximation and RPA for the spin and charge susceptibilities are suitable approaches for the case of weak interactions.